Sending signals through fiber optic cable is reliable and fast, but because of internal absorption and other effects, they will lose photons—which is a problem when the number of photons being sent is small. This is of particular concern in quantum networks, which typically involve a small number of entangled photons. Direct transmission through free space (vacuum or air) experiences less photon loss, but it's very difficult to align a distant receiver perfectly with the transmitter so that photons arrive at their destination.

A group in China has made significant progress toward solving that problem, via a high accuracy pointing and tracking system. Using this method, Juan Yin and colleagues performed quantum teleportation (copying of a quantum state) using multiple entangled photons through open air between two stations 97 kilometers apart across a lake. Additionally, they demonstrated entanglement between two receivers separated by 101.8km, transmitted by a station on an island roughly halfway between them.

Though the authors do not make this clear in the paper, their method is currently limited to nighttime communication. Nevertheless, their results achieved larger distances for multi-photon teleportation and three-point entanglement than before, and the tracking system used may even enable ground-to-satellite quantum communication—at least if it happens at night.

Quantum communication requires transmitting an arbitrary quantum state between two points, similar to how ordinary communication sends bits (voice or other data) across distances. However, a quantum state is a small amount of information, typically carried by a single photon, so many methods used in ordinary communication are out of the question (including broadcasting).

In fiber optic quantum networks, photon loss is large over significant distances, requiring the use of quantum repeaters. Point-to-point free-space transmission—either open-air or through the vacuum of space—is better, though larger distances allow the beam of photons to disperse. Atmospheric turbulence also contributes to photon loss in the air, with the losses increasing the farther the signal must travel.

One of the biggest challenges in point-to-point communication, however, is target acquisition by the transmitter and/or receiver. If the ground shifts slightly due to settling or tectonic activity, or atmospheric turbulence makes the receiver appear to move, the laser transmitting the signal can miss its target entirely. With few photons to spare in quantum communication, real-time tracking and acquisition is necessary. The researchers solved this problem using beacon lasers, bright beams that carry no information, but can be used to aim both transmitter and receiver, and wide-angle cameras.

As usual in quantum entanglement experiments, the group created entangled photons by stimulating a crystal with ultraviolet light. This produces a pair of photons with the same wavelength, but opposite (and unknown) polarization values. These entangled photons were subsequently sent to detectors, where their polarization quantum states were measured and compared. In the first experiment, one photon was sent 97km across Qinghai Lake (using a telescope to focus the beam), while the second was analyzed locally. Using these photons, the researchers copied the quantum state from the laboratory to the far station, achieving quantum teleportation over a much larger distance than previously obtained.

However, quantum communication sometimes also requires coordination between two distant receivers, so the researchers set up the transmitter on an island in the lake. The receivers were 51.2 and 52.2 km from the photon source respectively, on opposite shores of Qinghai lake, forming a triangle with the transmitter. The distance between the receivers—101.8km—was far enough to create a 3 microsecond delay between measurements of the photon polarization.

Given this setup, there was no possible way for the two receiving stations to communicate. Yet the photons they registered were correlated, indicating entanglement was maintained.

These experiments provide not only a proof of principle for free-space quantum communication, but also a means to test the foundations of quantum theory over larger distances than before. With very large detector separation, quantum entanglement experiments can help differentiate between standard and alternative interpretations of the quantum theory.

Though the long-distance aspect is promising, the fact that they set up on the shores of a lake (where no intervening obstacles exist) and that the experiment could only be performed successfully at night indicate its limitations. Author Yuao Chen told Ars via e-mail that they are working on solving the problem for daytime communication, but since the signal consists of single photons, it's not clear how this will work—the number of received photons fluctuated with the position of the Moon, so noise appeared to be a significant problem for them. Point-to-point communication will need to solve that problem as well before satellite-to-ground quantum networks are practical.

44 Reader Comments

Sounds like the Marconi radio experiment across the Atlantic. Very impressive. I wonder how long it will be till useful "transceivers" using this technology appear? How would one keep a quantum singularity or a q-bit in their pocket? If you drop your quantum phone will the other one rattle?

I would agree for vacuum, but for horizontal transmission through the atmosphere near the ground, I get (from googling around) a figure of 0.2 dB/km attenuation in clear weather (no wavelength specified), whereas optical fiber can have an attenuation as low as 0.01 dB/km (ZBLAN fiber at a 2 micron wavelength).

And this is ignoring the waveguiding property of optical fibers, which results in no spread of the signal (and thus no lowering of intensity due to spreading).

Marconi's experiment relied on the Ionosphere being able to bend the relatively long wave signals back to Earth on the other coastline. If he had tried the same experiment above 50 megaHertz, its unlikely he would have received the signal since the Ionosphere lets most VHF through microwaves frequencies right through it. Obviously laser guide beams in this experiment required line of sight. The experiment itself relied on the actual photons released by the entangled particle to be received at the receiver sites.

I thought entangled particles were only pairs. Perhaps they used 2 pairs of entangled particles. I'm going to have to read the original report.

This sounds like deep space communications would be a great use-case for the technology: much less thermal/gravitational/photonic interference and the benefit of breaking light speed limits would be that much greater.

Also: line of sight for radio transmissions depends on the frequency, the angle of incidence against the reflecting surface and the matter which intersects the radio wave. Low frequencies/longer wavelengths can reflect off atmospheric layers, while high frequencies do not. Ironically, "short wave" radio is actually a relatively low frequency/long wave for this purpose. Higher frequencies can still reflect off of solid and/or metallic surfaces, resulting in multi-path effects where a receiver receives multiple copies of a signal that took different reflective paths to reach the end point. These multi-path/reflection effects can be beneficial in cases where there is no LoS available, but generally cause more difficulty for the receiver when decoding/filtering the base signal.

I would agree for vacuum, but for horizontal transmission through the atmosphere near the ground, I get (from googling around) a figure of 0.2 dB/km attenuation in clear weather (no wavelength specified), whereas optical fiber can have an attenuation as low as 0.01 dB/km (ZBLAN fiber at a 2 micron wavelength).

And this is ignoring the waveguiding property of optical fibers, which results in no spread of the signal (and thus no lowering of intensity due to spreading).

Assuming the tunnel is straight and linear, you could easily aim a beam through it. If it has a bend in the middle, yes you'd need a mirror(s) (an Ionosphere) to bend the beam.

The Chunnel system in Europe use "wired" repeaters to relay broadcast signals to the riders and to repeat communications to the engineers driving the trains. Most of the tunnels in the US are too short to need to entertain the tunnel users.

From reading the abstract it looks like they had multiple photons emitted by the entangled pairs. The idea being to increase the signal to noise ratio (SNR) making the connection between the pairs more robust. The communication looks like it was basically statistical analysis instead of a meaningful human symbol or communication. Not a "Watson come here." phrase.

Quantum teleportation is not copying, it is the transfer of the quantum state from one particle to another. The original quantum state is destroyed in the process.

Copying (or "cloning") of a quantum state is forbidden in quantum mechanics. This is not a trivial nit-pick; it is the sole reason why quantum key distribution for cryptography is secure: any attempt at copying by an eavesdropper introduces a detectable rise in noise.

The US and the Russians have very reliable radar systems that detect over the horizon targets. If you blast enough RF power out in bursts, you can receive fairly reliable signal returns off aircraft using ordinary ionized air as your "radio mirror". In other words ranges in the hundreds of miles, far beyond the horizon. Typically the horizon is 20 to 40 miles away depending on topology. There is a US radar station in Alaska that keeps an eye on the eastern coastline of Siberia, perhaps as far south as Japan.

Radio amateurs the world over get blasted by the Russian "woodpecker" since that's what it sounds like on the HF bands. The US system has a tendency to stay out of the amateur HF radio bands since they tend to be fairly vocal with their elected politicians. Also the US system is directed away from the continental US. Obviously, the Russian system is pointed at the US. The changes made in frequency can be used to more carefully define the appearance of the remote target.

The relationship to the experiment...If you send enough signals, you'll get returns even in high noise environments.

i was under the impression that once the particles were entangled, line of site was irrelevant. I thought it didn't matter where in the universe the other particle in the pair actually was. I was always under the impression that the real challenge was creating and maintaining that entanglement and getting each particle of the pair to their destinations reliably.

If that's the case, then it seems like what we need is the tech to contain and maintain the entangled particles reliably for long periods of time.

Quantum teleportation is not copying, it is the transfer of the quantum state from one particle to another. The original quantum state is destroyed in the process.

I agree. I do not claim to understand quantum mechanics in its entirety. At least from what I've read & understand, what the Chinese scientists were trying to do was to transfer the Q-state using emitted photons from one particle to another via a beam of the actual photons emitted. The entanglement occurs when the photon is absorbed by the second particle. (I think)

I was under the impression that entanglement required something to link the two particles together and then when the particles separated, what happened to one would appear to cause something to happen to the other at the same instant. Is that right?

As to the issue of quantum security devices. I know that NASA has a system in use that can detect when a new device is connected to their network(s) even if the device is totally unpowered. It evidently is based on quantum mechanics since it uses the noise on the system to determine a new connection or the change of connection status. I would suspect though that it is a management nightmare because simply disconnecting a nic or changing out a CAT 5 cable would trip the alarm. There are engineers at every second desk and they likely move things to suit the moment. A TDR is probably used next to determine the path to the newly connected device.

I wouldn't recommend adblocking Ars since they rely heavily on ads to support the site. I voiced my complaints about this earlier. It's a bug that they are tracking down. Aurich has already been apologizing for the issue and has stated he and others are looking into it.

This sounds like deep space communications would be a great use-case for the technology: much less thermal/gravitational/photonic interference and the benefit of breaking light speed limits would be that much greater.

That's what I thought at first read-through. Instant communication with the Curiosity rover would have made it even more exciting. Unfortunately, it appears that this won't work out well (http://en.wikipedia.org/wiki/No-communication_theorem), because (as usual) math gets in the way of awesome Sci-Fi technology. I can't pretend that my knowledge of quantum physics is solid, so I may have misread it, but it appears that a Quantum channel cannot be used to communicate classical information, and vice versa.

We'd appreciate that the discussion of ad-blocking solutions take place somewhere other than Ars. As a poster above mentioned, we need ad revenue, and if you don't want to see ads on Ars, we offer subscriptions to avoid them.

I can't pretend that my knowledge of quantum physics is solid, so I may have misread it, but it appears that a Quantum channel cannot be used to communicate classical information, and vice versa.

You can still send morse code with quantum transmission or binary or digital or more later.

How would that work? If you're only "sending" the quantum state, you would still have to measure it to get a usable bit. From what I've read, you still need to send some classical information to recover any quantum qubits that are needed (http://en.wikipedia.org/wiki/Quantum_te ... on#Remarks is what gives me this impression). It seems like this could be incredible for sending encrypted messages over previously created channels, but in terms of superluminal communication, is essentially useless.

This sounds like deep space communications would be a great use-case for the technology: much less thermal/gravitational/photonic interference and the benefit of breaking light speed limits would be that much greater.

Also: line of sight for radio transmissions depends on the frequency, the angle of incidence against the reflecting surface and the matter which intersects the radio wave. Low frequencies/longer wavelengths can reflect off atmospheric layers, while high frequencies do not. Ironically, "short wave" radio is actually a relatively low frequency/long wave for this purpose. Higher frequencies can still reflect off of solid and/or metallic surfaces, resulting in multi-path effects where a receiver receives multiple copies of a signal that took different reflective paths to reach the end point. These multi-path/reflection effects can be beneficial in cases where there is no LoS available, but generally cause more difficulty for the receiver when decoding/filtering the base signal.

I don't think you can use this for an ancible bc no useful info can be transmitted(I.e.random bits).

Assuming the tunnel is straight and linear, you could easily aim a beam through it. If it has a bend in the middle, yes you'd need a mirror(s) (an Ionosphere) to bend the beam.

The Chunnel system in Europe use "wired" repeaters to relay broadcast signals to the riders and to repeat communications to the engineers driving the trains. Most of the tunnels in the US are too short to need to entertain the tunnel users.

Perhaps (really, I don't know), but the BART tunnels are, and as we found out, communications work only when BART wants them to.

Quote:

From reading the abstract it looks like they had multiple photons emitted by the entangled pairs. The idea being to increase the signal to noise ratio (SNR) making the connection between the pairs more robust. The communication looks like it was basically statistical analysis instead of a meaningful human symbol or communication. Not a "Watson come here." phrase.

Me, I wonder how this would work different if the photons were just created with whatever polarization.

I read an article, I think in Scientific American a few years back where the intrepid heros used an entangled pair (or a few pairs) to send a signal to (or from, don't recall) Mars. It was really rather stupid as the intrepid hero had to make a WAG (he guessed right of course) to save the day.

I wouldn't recommend adblocking Ars since they rely heavily on ads to support the site. I voiced my complaints about this earlier. It's a bug that they are tracking down. Aurich has already been apologizing for the issue and has stated he and others are looking into it.

I have to admit that I was completely jazzed to see a tiny Curiosity flitting around the site when I logged in just now. And yes, I ended up with a Siemens window. If it's never anything but tiny space probes, and doesn't happen on a continual basis, I'm okay with it.

I thought any radio waves of a sufficient power to be usable did suffer from the line-of-sight problem?

Tune your AM radio to 7,480 or 11,570. Enjoy North Korea propaganda.

I sincerely hope you don't have a line of sight to their antenna.

The higher the frequency the more the broadcast requires line of sight. Visible light has very high frequencies of course, so it is always line of sight (reflecting or refracting notwithstanding).

Lower frequencies such as shortwave can propagate around the world.

"If you build it, they will come." I dunno about line-of-sight (actually I do quite well!) but if you throw enough power out there, you can get enough signal to bend well over the horizon. While operating off the southern coast of California, we were waiting for a B-52 to come play with us (my destroyer). I kept our TACAN (TACtical Air Navigation) beacon in near perfect alignment. I know because that was the first thing I did after muster each morning. Well, right after I got my first coffee, it being the Navy and all with those atrocious hours ;-). The B-52 took off from their air-force base in Colorado and climbed up to 50,000'. When the navigator punched in our frequency (right at 1 GHz), they picked up my signal right away. Amazing what you can do when you toss 20,000 watts of RF out there. [I received a huge 'Attaboy' as well as a letter of appreciation for that one.] LOS can sometimes be only a suggestion, not a law of nature!

I can't pretend that my knowledge of quantum physics is solid, so I may have misread it, but it appears that a Quantum channel cannot be used to communicate classical information, and vice versa.

You can still send morse code with quantum transmission or binary or digital or more later.

How would that work? If you're only "sending" the quantum state, you would still have to measure it to get a usable bit. From what I've read, you still need to send some classical information to recover any quantum qubits that are needed (http://en.wikipedia.org/wiki/Quantum_te ... on#Remarks is what gives me this impression). It seems like this could be incredible for sending encrypted messages over previously created channels, but in terms of superluminal communication, is essentially useless.

This is correct. Superluminal communication via quantum teleportation is not possible (the wilipedia link is correct). In order to reconstruct a teleported qubit at one end, additional information must be sent from the other end by standard means. I am somewhat confused as to what the communications aspect of this experiment is - unless it is something to do with quantum cryptography.

I thought any radio waves of a sufficient power to be usable did suffer from the line-of-sight problem?

Tune your AM radio to 7,480 or 11,570. Enjoy North Korea propaganda.

I sincerely hope you don't have a line of sight to their antenna.

7,480KHz and 11,570KHz belong to "열린북한방송(Open North Korea Broadcast)", which is a South Korean broadcast intended to send news (or South Korean propaganda, depending on your view) to North Korean citizens - basically, to make them "open" their ears.

Just because the name has North Korea in it doesn't mean it's FROM North Korea.

We'd appreciate that the discussion of ad-blocking solutions take place somewhere other than Ars. As a poster above mentioned, we need ad revenue, and if you don't want to see ads on Ars, we offer subscriptions to avoid them.

Thanks!

And it has been incredibly effective. I haven't seen a single advert since subscribing, highly recommended.